SOOO I caught him at it again, after a good month of not touching his tail I saw Aquarius take a big bite out of his tail and now he's missing a chunk. I threw him into a hospital tank with some AQ and stresscoat to help it heal again but WHAT CAN I DO TO STOP HIS TAILBITTING???? It started perhaps two months after I first got him and then I got an ADF and added him in the tank, the tailbitting stopped for a month and then it started again so we rearanged his tank and it stopped for two months, then it started again but we were moving to another state and so we moved, set up his tank and it stopped. Now it's started again today and I am getting sick of watching him turn his beautiful tail into a mess. What can I go to stop him from doing this? Is it a boredom thing? Do I need to change his environmental constantly and add new things?

Sorry to hear of him biting again! It's pretty annoying. He is a halfmoon, is he not? I would say the weight of his tail has some influence on his biting- even without a filter. My sister's HM is in a filter-less tank and bites all the time.
You can work with the space here, with a heavy finned betta you can add more to the tank without affecting his quality of life. I would just block that thing with plants. Then he can play jungle and explore them and then rest anywhere if he needs to.

I've tried:
Increasing / decreasing temp.
Increasing / decreasing light.
Adding / removing / changing around decorations, silk plants, and live plants.
Putting him near another male betta / hiding him from male bettas.
Putting him near a female betta / hiding him from the female betta.
More attention / less attention.
Changing the brand of his pellets.....
And so on....

But all he wants to do is eat his tail. I've even found bits of it in his poop!

I've joked on another thread that I need to find a fish-sized e-collar for him!

Yeah he is a DTHM and since I first got him his tail got HUGE. Well now it's little since he RIPPED OFF A CHUNK OF TAIL AAAAAAARRRGGGHHHH

I'm trying the heavily planted hospital tank idea next to my girl. I was wondering if it was since my other male Betta moved in next to him but Hollow is losing his sight and ignores Aquarius and Aquarius for the most part ignores him aside from when he stops to watch him. ALSO now that my betta is out of his tank my adf refuses to eat and today is his feeding day. He did this a few days ago when I removed Aquarius from the tank to feed him and he just straight up refused to eat without the fish in there. Going to add in his moss ball too, see how he does :/ Hope he leaves his tail alone in the hospital tank.